Serum antibody decay following RSV infection in adults was examined to evaluate the durability of the immune response. Twenty subjects with RSV infection and 10 subjects who remained RSV uninfected had blood samples obtained over 16-25 months analyzed by microneutralization assay and enzyme immunoassay. The mean titers of infected subjects rose approximately eightfold post-infection. The mean rate of antibody decline was -0.20 log 2 titer per month which led to a > or =fourfold drop in titer in 75% of subjects at 1 year. In contrast, titers of uninfected subjects were relatively stable. The partial immunity resulting from a boost in serum antibody following natural RSV infection in adults appears to be short lived.